FR2462791A1 - SUBMILLIMETRIC WAVE DETECTOR COMPRISING MULTIPLE NON-LINEAR RESPONSE ELEMENTS - Google Patents

Abstract

FAST SUBMILLIMETRIC WAVE DETECTOR INCLUDING A FLAT ARRAY OF ELEMENTS11 ON A NON-LINEAR RESPONSE SEMICONDUCTOR. THE DETECTOR INCLUDES A FLAT CURTAIN OF 10 PARALLEL LONG ANTENNAS, ASSOCIATED WITH THE NON-LINEAR RESPONSE DETECTOR ELEMENTS BY PUNCTUAL CONTACTS. APPLICATIONS TO THE CONSTITUTION OF VIDEO DETECTORS AND HETERODYNE SYSTEMS.

Description

The present invention relates to detectors usable in the range

  submillimetric waves, practically in the range of 10 microns to 1 millimeter. The current fast detectors usable in this range, which belongs to both the far-infrared and the microwave domains, are few and insensitive. Virtually all of them use Schottky diodes as nonlinear elements which have the advantage of having a large bandwidth, of being fast and of operating at ambient temperature. But the very small surface that must be given to Schottky diodes (less than one micron square) requires associating them with a power structure. High losses in

  conducting materials at submillimeter wavelengths

  prohibit the use of metal waveguides

  and lead to the use of antennas as feed structures

mentation.

  In a known detector, the feed structure

  tion for coupling the incident radiation and the Schottky junction is carried out using a finished wire

  by a point which rests on the anode of the diode.

  The antenna is constituted by the portion of the wire between the tip and a high-curvature portion which acts as a support spring of the wire on the anode. The coupling between the long antenna thus produced and the non-linear element constituted by the Schottky diode is unsatisfactory. Indeed, the antenna has several lobes of emission and reception and the radiation pattern has a symmetry of revolution. As a result, only a fraction of the main lobe can be illuminated by incident radiation, highly directional at this frequency, which decreases sensitivity, and some of the energy is re-transmitted

  on the lobes other than the one that is illuminated.

  The present invention aims to provide a detector usable in the submillimeter range, better than those previously known to the requirements of the practice, in particular having a much higher sensitivity all

by remaining of simple construction.

  For this purpose, the invention proposes in particular a detector comprising a plane array of elements on semicon-. nonlinear response conductor and a plane curtain of long parallel antennas associated with the detector elements at

  nonlinear response by point contacts.

  Non-linear response elements will gener-

  normally constituted by Schottky diodes distributed under

  form of a matrix network on a semiconductor wafer

  composite conductor, such as gallium arsenide.

  As long as epitaxially grown and doped arsenide of epitaxy is used, so that the conduction is carried out thermo-ionically and not by tunnel effect, it is possible to

  perform satisfactory operation up to a frequency of

about 3 THz.

  It should be noted that the increase in

  obtained thanks to the implementation of the invention

  not simply because of the increase in the number of

  detector and associated antennas, but above all the radical transformation of the radiation pattern by suppressing the symmetry of revolution and reinforcement of one main antenna lobe at the expense of the others. When using the detector, it will obviously be oriented so that the direction of the incident radiation coincides

  approximately with the axis of the main lobe.

  The antenna plane curtain is likely to

  many practical achievements. All should tend to provide a curtain whose antennas are correctly

  connected to the anodes of the diodes, are perfectly parallel

  and are regularly excluded at the level of

contacts with the diodes.

  A first solution is to constitute the curtain by a sheet of parallel son each of which is brought into contact with the corresponding detector element by

  dull point, but it is difficult to maintain

  a satisfactory parallelism of the wires without drowning them in a dielectric support. However, most of the dielectrics currently used commercially are absorbing in a broad band of the spectrum, hence a loss of sensitivity. Another solution consists in producing the sheet of wires in the form of thin linear conductors constituted by photoengraving on an insulating thin layer

  covering the semiconductor medium in which are

  the nonlinear response elements. Such a detection

  It has the advantage of being suitable for manufacturing by Planar techniques widely used in the field of integrated circuits. Yet another solution consists in producing the curtain in the form of a plate-shaped sensor (which amounts to an infinity of infinitely close wires), a plate coupled to the non-linear response elements by distinct points. Experience has shown that, in this case, good results are obtained by giving the plate sensor a length-corresponding to a few wavelengths and a width whose length ratio is

  between 3.5 / 5 and 4/5. The spikes belong to

  tageusement to parallel son solidaires electrically and mechanically of the plate, arranged symmetrically with respect to the median plane thereof, the two extreme son

  being arranged along the lateral edges.

  Finally, to further reduce the sidelobes and strengthen the main lobe, we will usually be interested

  to add to the antenna curtain one or more reflectors.

  Several reflectors can be combined with a grid

  input to form a tuned resonant cavity.

  The invention is capable of many applications

  cations. In particular, it makes it possible to detect

  submillimetric video-frequency radiation units and mixers providing a medium frequency from submillimetric waves and a harmonic of a local source, such as a phase-referenced klystron. The sources of radiation that can be used include lasers, for example with hydrocyanic acid emitting on 0.89 THz, carcinotrons and gyrotrons which have the advantage of providing

higher power.

  The invention will be better understood when reading

  the following description of detectors constituting

4 _ 2462791

  particular modes of realization given as examples

  non-limiting ples. The description refers to the drawings

accompanying him, in which:

  FIG. 1 is a schematic diagram of a first

first embodiment,

  FIG. 2 is a large scale diagram of

  trant the junction between a tip and the anode of a Schottky diode, - Figure 3, similar to Figure 1, shows a variant,

  FIG. 4 is a schematic perspective view

  In another embodiment, FIG. 5, similar to FIG. 1, shows another variant using a plate sensor. FIG. 6 is a diagram showing the combination of impedances in the mounting of FIG. Figs. 9 and 8 show two possible arrangements of reflectors; Fig. 9 shows a possible practical arrangement of the detector, in section along the line IX-IX of Fig. 11; Fig. 10 shows on a large scale an area in FIG. a circle in phantom in Figure 9, - Figure 11 is a section along the line

XI-XI of Figure 9.

  In its simplest embodiment, shown in FIG. 1, the detector according to the invention comprises n wires arranged in parallel, at equal distances, forming a curtain. Each of the wires 10 has a blunt tip-shaped end of contact with a corresponding Schottky diode 11. The end portion opposite the tip is provided with a loop 12 forming a spring and connects

  to a driver 13 connected to the output. Each wire is equi-

  are worth to a wire antenna whose length L corresponds to the distance between the. tip and the curvature change of the

wire, at the end of the loop.

  We know that Schottky diodes, constituted

  essentially a metal in intimate contact with a semi-

  conductor without interposition of oxide layer, give satisfactory results in submillimetric waves at -5-

  condition of having a very small surface area and of being

  on a semiconductor of very high mobility. Such diodes can be formed in the form of a network on a semiconductor support made of gallium arsenide. For this, we start from a support comprising a substrate N 14 gallium arsenide (Figure 2) on which was grown by epitaxy a thin layer of N 15 gallium arsenide.The layer 15 is deposited on thin film (about 1000 A in

  general) of insulating silica 16. By masking and microlitho-

  electronic graph or X, then windows are made exposing the layer 15, on a surface not exceeding 1 pm. Finally, the bottom of the windows is metallized

  performed to form the anodes.

  The wires will usually be tungsten. Their diameter may be difficult to be less than 10 m. The

  terminal point may in particular be formed by electro-

  lysis in an alkaline solution (for example 3N NaOH)

  or acid (for example 1N HNO3 + 1N HF).

  The detector thus produced has a radiation pattern having a maximum in the median plane 17 of the curtain (FIG. 1). The maximum sensitivity is obtained in a direction at an angle 9 with that of the wires, which corresponds approximately to the formula sin 0 v 0.43 V ^^ / L In this formula, 0 is the wavelength

radiation.

  The sensitivity of the detector increases proportionally

  at {E., where n is the number of diodes and wires, at least as long as the wires are parallel,

equidistant and all connected.

  The provision that has just been described presents difficulties of realization. Indeed, it is difficult to make a curtain in which all the anodes are

  correctly connected to the wires, all wires are perfectly

  parallel (a flexion occurring during contact)

  and all the wires are equidistant.

  A first solution to overcome this difficulty is shown schematically in Figure 3. It consists of trapping the son 10a in a thin support 18 transparent dielectric that holds them in place. The wire curtain can then be manufactured by winding a continuous filament on a flat mandrel, with a constant and weak pitch, and then molding the rectilinear portions of the winding in the

  sheet 18 before cutting the winding following two gen-

  to form the wires 10a. The wires are then electrolytically treated together at one end

  to form blunt contact tips.

  This solution, satisfactory from the mechanical point of view, has the disadvantage of requiring the use of dielectrics having sufficient transparency in the submillimeter range. Now, dielectric plastics

  commonly used at present (polytetrafluoroethylene)

  ethylene, polyethylene, etc.) have an absorption which is not

  negligible and results in loss of sensitivity.

  Another solution is to implement a technique approaching that used for the realization of integrated circuits. It allows to arrive at a monolithic structure associating the diodes and the network of antennas, such as that illustrated schematically

in figure 4.

  In this case, the non-linear response elements and the antenna curtain can be realized in a single operation sequence. If it is assumed that the non-linear elements are Schottky diodes, one starts from a wafer 14b of gallium arsenide coated with a layer 15b obtained by epitaxial growth. We realize on this layer, by

  a usual technique of manufacture, an insulating layer

  SiO2 lant, 16b. On a fraction of the wafer,

  is, through the insulation, by electro-photogravure

  nique or X (given the very small widths to achieve), a honeycomb network of Schottky diodes llb. On the insulator 16b, then is deposited, by photoetching, a metal (gold or aluminum in general) in the form of thin strips to connect several diodes in parallel. This is a series of parallel bands playing the

role of antenna curtain.

2b62791 -7-

  It is thus possible to solve in a simple way

  mechanical resistance problems. The currently available photoetching techniques make it possible to achieve low strip dimensions and diode areas. If such techniques are used, the distance between

  diodes llb is no longer limited by technological imperatives.

  but physical. It is indeed necessary that the anodes are sufficiently far apart from each other not to flow into one another, which is to say that 1 'the gap between the active anodes, that is to say say connected,

  must be large in front of the diffusion length of

  minority players in semiconductor 15b. In practice, the spacing should not be less than 15 microns for gallium arsenide. But, mask positioning problems make exact alignment difficult and then impose a diode distribution step of at least 15 pm to be certain that all the wires will

are connected.

  All the solutions envisaged so far use a curtain of discrete antennas. But we can also merge them into a single plate-shaped sensor, equivalent to antennas infinitely close together. The arrangement is then that shown diagrammatically in FIG. 5, comprising a plate proper 17 extending by points 18. These points may be constituted by the terminal portions of interconnected wires.

  of the plate or by extensions of it when

  that it is carried out by photogravure. The length L of

  the plate will always be of several wavelengths.

  As a result, the main antenna lobe will not be perpendicular to the curtain. A systematic study of the influence of the ratio l / L of the width to the length of the antenna has shown the presence of a maximum which, for

  L = 5 X (X being the wavelength), is relatively flat.

  In practice, satisfactory results with regard to the importance of the main lobe with respect to the side lobes are obtained for l / L of between 3.5 / 5 and 4/5. The width of the main lobe is reduced to about 250 in a plane parallel to the length of the plate

  17 and perpendicular to this plate. The symmetry of revolution

  The solution is completely broken, and with five evenly distributed yarns, a main lobe of about 250, much larger than the secondary lobes, is obtained. It should be noted in passing that, because of the

  very high frequency, the currents are distributed

  on the edges of the plate, in the hatched schematic zone in FIG. 5. Consequently, it will always be advantageous to arrange the two extreme points

  18 in the extension of the lateral edges.

  The amplitude of the main lobe increases substantially as n (where n is the number of threads or spikes). As for the amplitude of the second lobe, it seems to saturate as soon as we exceed n = 5 approximately. In practice, it appears

  5 threads or spikes already give satisfactory results.

  and the increased complexity is not justified by

  an appreciable increase beyond ten sons.

  When the number of wires or spikes is high, the diagram equivalent to the system is that given in FIG. 6. It comprises several charges Z1, Z2 ... Zn constituted by the anodes connected in parallel, of decreasing

  the total impedance that charges the generator. We design-

  ra by Za the impedance associated with the generator equivalent to

the antenna.

  The adaptation will be optimal and will result in

  a maximum sensitivity if Za = Z, equivalent impedance

  a eq slow to all loads, given by -1 n 1 eq i Impedance Za depends on frequency and dimensions. Zi impedances also depend on the

  frequency and dimensions, as well as doping.

  In each case, an approximation can be made by calculation. By way of example, it may be noted that an optimum was obtained at 1 THz for the following values of the parameters: n = 7 wires L = 1.7 mm, or approximately 5? 1 = 1.35 mm, ie approximately 4X (A = 337 microns) diameter of the diodes - 0.8 micron epitaxy thickness 1500

carrier density N = 1017 / cm3.

  The sensitivity obtained was of the order of 27 volts

  by Watt, without the use of any reflectors.

  In order to reduce the influence of the secondary lobes which remain in the antennas which have just been described, it is advantageous to add them to one or more reflectors which modify the antenna radiation pattern, in particular by restricting the range of

visibility of it.

  Among the antenna arrangements that can be used, those shown schematically in FIGS.

  three reflectors. In the case illustrated in Figure 7, use

  With two parallel reflectors 20 and 21, parallel to the arrival direction F of the incident radiation, the antenna curtain is placed at an angle θ (FIG. 1) with respect to F, the end of the curtain connected to the response elements.

  nonlinear being near the third reflector 22.

  Optimization can then be carried out by adjusting successively

  the distance 11 between the reflector 20 and the tips, then the distance 12 between the reflector 21 and the other end of the curtain, and finally the distance 13 'As an example, it can be noted that for 1 = 0.98 X, we found a maximum of sensitivity for 12 = 4.58 X. For these two values finally, the optimum was reached for 13 = 1.76 X

  X being the incident wavelength.

  In the arrangement shown in FIG. 8, the reflector 20 is parallel to the direction of the radiation

  incident F and the reflector 21 parallel to the curtain of

  tenne. The optimization can then be done by successively adjusting x1, x2 and x3 (the third reflector is not essential). The optimum was found to be substantially x1 = 0.98? and x2 then equal to 0.588?

246219 1

- 10-

1.609A or 2.693A.

  One or the other of the above two arrangements makes it possible to multiply the amplitude of the detected signal by a factor which is in general between 5 and 8; The detectors that have just been described are capable of numerous practical embodiments, one of which, including reflectors, is shown in FIGS.

9 to 11.

  The device shown in the figures comprises a casing 23 in several assembled parts, in which are drilled two perpendicular bores 24 and 25 concurrent. The actual detector is placed in the chamber 29 common to both bores. The device is designed to be oriented, during operation, so that the incident radiation F is in the axis of the bore 25. In this bore slides a reflector 22d movable with a threaded rod 26-in a cap 27, using a nut 28. Two reflectors 20d and 21d are similarly mounted to be movable to

  face in the bore 24. There may be further reflection

  additional factor 30 (Figure 11) in a third

  bore perpendicular to the two previous ones.

  The detector is carried by a guide rod 31 (FIGS. 9 and 10) guided in a passage of the housing forming, with the axis of the bore 25, the angle O. The rod 31 is adjustable in position using an adjustment screw 32.

  It will be assumed that the detector is of the illus-

  Figure 5. The plate sensor 17 is connected to an output socket 33 located in the extension of its edge remote from the diodes. It is connected to diodes, worn by

  the rod 31, by a series of parallel points.

  To work in narrow band, while Schottky diodes are broadband detectors,

  it will be interesting to place it in a resonant cavity.

  For this purpose, there are provided, in the opening of the bore 25 which receives the incident radiation, grids 34 whose

  distance is adjustable by means not shown.

  These two grids play a role of interferometer 2fr52791

  Fabry-Perrot, adjusting the input transmission and

  put to constitute a multi-mode cavity whose coupling with the sensor can be adjusted by keys until the

critical value.

  FIGS. 9 to 11 correspond to a provision of reflectors according to the diagram of FIG. 7. However, the bores can be arranged in such a way that the receivers reproduce the diagram of FIG. 8. Whatever the embodiment adopted, we see that we obtain an open structure significantly increasing the sensitivity of the detector and reducing the radiation pattern to practically a single lobe, whose angular development

is of the order of 200.

  As mentioned above, the device can be used as a video frequency detector, with a flat-line microwave connector output. A tee then makes it possible to connect the antenna to a continuous source of polarization of the diodes. It can also be used

in heterodyne.

  The invention is capable of many embodiments other than those described by way of

  examples. In particular, one can use a semi-

  conductor other than Ga As, provided that the mobility of the carriers is of the same order or higher. We can cite

  Inx As Ga1 x doped in tellurium, Zn As, In Sb and Hg Cd Te.

  IxAG 1-x It goes without saying that the invention extends to

  all variants remaining within the framework of equivalences.

-12 - 2462791

Claims (10)

  1. Rapid submillimetric wave detector, comprising a flat array of semiconductor elements with a non-linear response, characterized in that it comprises a flat curtain   long parallel antennas associated with the detecting elements   non-linear response by point contacts.   2. Detector according to claim 1, characterized   characterized in that the curtain is constituted by a sheet of parallel wires in contact with detector elements   blunt tip or line conductors   areas made by photogravure on a thin insulating layer covering a semiconductor support on which   nonlinear response elements are constituted.   3. Detector according to claim 1, characterized in that the curtain is constituted by a plate-shaped sensor coupled to the response elements.   nonlinear by distinct points.   4. Detector according to claim 3,   characterized in that the plate has a corresponding length   at a few wavelengths and a width   between 3.5-fifths and 4.5-fifths of the length.   5. Detector according to claim 3 or 4, characterized in that the tips belong to parallel son electrically and mechanically integral with the plate, projecting from it, arranged symmetrically with respect to the median plane of the plate, the two extreme wires being arranged along the edges side of the plate.   6. Detector according to claim 3, 4 or, characterized in that the characteristics of the plate sensor are chosen so that its impedance Za is conjugated with the impedance Zeq equivalent to all the charges constituted by the non-response detector elements. linear, given by: n 1 = i 1 Zeq i Zi   Zi being the elementary impedance of a nonlinear element   area and n the number of these elements. - 13 -   7. Detector according to any one of the   preceding dications, characterized in that the nonlinear response detector elements consist of Schottky diodes on compound semiconductor, advantageously Ga-As, whose spacing between them is at most equal to 15 microns.   8. Detector according to any one of the   previous indications, characterized in that it comprises five   to ten detector elements or detector element wires.   9. Detector according to any one of the   preceding dications, characterized in that it comprises at least one reflector for modifying the diagram of the antenna curtain.   10. Detector according to claim 9, characterized   in that the antenna plane curtain is placed in a cavity delimited by said reflector and at least one entrance gate.